1. Field of the Invention
The present invention relates in general to a motor vehicle braking system of diagonal or X-crossing type having two pressure application sub-systems each including three solenoid-operated valve devices which are electrically controlled to regulate braking pressures in front and rear wheel brake cylinders. More particularly, the present invention is concerned with improvements in techniques for controlling such solenoid-operated valve devices.
2. Discussion of the Related Art
The assignee of the present invention developed a braking system of diagonal type for a motor vehicle, wherein each of two pressure application sub-systems includes three solenoid-operated valve devices electrically controlled to regulate braking pressures in a front wheel brake cylinder and a rear wheel brake cylinder. This braking system was developed in an effort to reduce the required number of the solenoid-operated valve devices for controlling the braking pressures for the four wheels of the motor vehicle.
The braking system developed by the assignee is disclosed in U.S. Pat. No. 5,538,334 to Kushi et al. corresponding to JP-A-223529, which is a laid-open publication of unexamined Japanese Patent Application filed in the name of the assignee. The arrangement of this braking system will be briefly described referring to FIG. 29.
This braking system is designed for use on a four-wheel motor vehicle having front right and left wheels and rear right and left wheels. The braking system has two pressure application sub-systems connected to respective two mutually independent fluid pressurizing chambers of a master cylinder 200. One of the sub-system includes a brake cylinder for braking a front right wheel and a brake cylinder for braking a rear left wheel, and the other sub-system includes a brake cylinder for braking a front left wheel and a brake cylinder for braking a rear right wheel. Thus, the braking system is referred to as "diagonal" or "X-crossing" type in the present application. The two pressure application sub-systems are identical in construction with each other. One of these sub-systems is shown in FIG. 29.
In each pressure application sub-system, the corresponding pressurizing chamber of the master cylinder 200 is connected through a front brake cylinder passage 204 to a front wheel brake cylinder 202 for the front wheel Fr. A rear brake cylinder passage 206 is connected at one end thereto to the front brake cylinder passage 204 and at the other end to a brake cylinder 208 for the rear wheel brake cylinder 208. A normally open first solenoid-operated valve device 212 having an open state and a closed state is disposed in a portion of the front brake cylinder passage 204 between the master cylinder 200 and a point of connection between the front and rear brake cylinder passages 204, 206. A normally open second solenoid-operated valve device 214 also having an open state and a closed state is disposed in the rear brake cylinder passage 207. A reservoir passage 218 is connected at one end thereto to a reservoir 216 and at the other end to a portion of the rear wheel brake cylinder 206 between the second solenoid-operated valve device 214 and the rear wheel brake cylinder 208. A third solenoid-operated valve device 220 having an open state and a closed state is disposed in the reservoir passage 218. A pump passage 224 is connected at one end thereof to the reservoir 216 and at the other end to a portion of the rear brake cylinder passage 206 between the second solenoid-operated valve device 214 and the point of connection of the front and rear brake cylinder passages 204, 206. However, the pump passage 224 may be connected to a portion of the front brake cylinder passage 204 between the first solenoid-operated valve device 212 and the front wheel brake cylinder 202. A pump 222 is disposed in the pump passage 224, for pressurizing a fluid received from the reservoir 216.
The braking system includes a controller 226 which is adapted to selected one of a plurality of pressure control modes for each wheel brake cylinder, on the basis of the slipping condition of the corresponding wheel. The pressure control modes includes a pressure decrease mode, a pressure increase mode and a pressure hold mode in which the braking pressure in the wheel brake cylinder is decreased, increased and held constant (raised, reduced and maintained), respectively. In an anti-lock braking pressure control operation, the controller 226 activates an electric motor 228 for driving the pump 22, and suitably controls the solenoid-operated valve devices 212, 214, 220, to regulate the braking pressures in the wheel brake cylinder 202, 208 of each pressure application sub-system, so as to prevent an excessive amount of slip of each wheel during braking of the vehicle.
In this braking system, the pressure increase mode includes a pump increase mode and a master cylinder increase mode. The pump increase mode is available when a sufficient amount of brake fluid is stored in the reservoir 216, while the master cylinder increase mode is available when the amount of the brake fluid stored in the reservoir 216 is not sufficient. In the pump increase mode, the controller 226 closes the first solenoid-operated valve device 212, and activates the motor 228 to operate the pump 222, so that the braking pressures in the front and rear wheel brake cylinders 202, 208 are increased by the pressurized fluid delivered from the pump 222. In the master cylinder increase mode, the controller 226 places the first solenoid-operated valve 212 in the open state, so that the braking pressures in the wheel brake cylinders 202, 208 are increased by the pressurized fluid delivered from the master cylinder 200.
In this braking system wherein each pressure application sub-system uses the three solenoid-operated valve devices 212, 214, 220, the total number of the solenoid-operated valves required for controlling the braking pressures in the four wheel brake cylinders of the vehicle is as small as six. Another advantage of this braking system is derived from the arrangement in which the fluid stored in the reservoir 216 is pressurized by the pump 222, and the pressurized fluid is returned to the portions of the front and rear brake cylinder passages 204, 206 which are isolated from the master cylinder 200 by the first solenoid-operated valve device 212 placed in the closed state. In other words, the fluid in the reservoir 216 need not be returned to the master cylinder 200 whose pressure is higher than the pressure in the above-indicated portions of the front and rear brake cylinder passage 204, 206 which are located downstream of the closed first solenoid-operated valve device 212. Accordingly, the required delivery pressure of the pump 222 need not be equal to or higher than the pressure of the master cylinder 200, whereby the required capacities of the pump 222 and the motor 228 can be significantly reduced.
On the other hand, however, the braking system described above suffers from a problem that it is difficult to control, as needed, the rate of change of the braking pressure in each wheel brake cylinder.
In each pressure application sub-system using the three solenoid-operated valve devices, there are only eight combinations of the operating states (open and closed states) of the solenoid-operated valve devices, as indicated in the table of FIG. 9. Accordingly, there are only eight combinations of the pressure control modes for the front and rear wheel brake cylinders. Each combination of the operating states of the solenoid-operated valve devices will be referred to as a control state of the valve devices. That is, there are a total of eight control states Nos. 1-8 of the valve devices.
Described in detail, the combinations of the open and closed states of the three solenoid-operated valve devices do not permit a sufficiently large number of pressure control modes, which include, for instance, a hold-hold mode for the front and rear wheel brake cylinders, a slow front pressure decrease mode for the front wheel brake cylinder, and a slow rear pressure decrease mode for the rear wheel brake cylinder. In the hold-hold mode, the pressure hold mode is selected for both of the front and rear wheel brake cylinders. In the slow front pressure decrease mode, the braking pressure in the front wheel brake cylinder is reduced at a lower rate than in the pressure decrease mode established in the control state No. 7 indicated in the table of FIG. 9. In the slow rear pressure decrease mode, the braking pressure in the rear wheel brake cylinder is reduced at a lower rate than in the pressure decrease mode established in the control states Nos. 3, 5 and 7 also indicated in the table of FIG. 9.
The combinations of the operating states of the three solenoid-operated valve devices indicated in the table of FIG. 9 includes the combination established in the control state No. 6, which is provided to establish the pressure hold mode for the front wheel brake cylinder 202. Actually, however, the braking pressure in the front wheel brake cylinder is slowly increased, when the first and third solenoid operated valve devices 212, 220 are closed while the second solenoid-operated valve device 214 is open. That is, the braking system does not include a solenoid-operated valve device for disconnecting the front wheel brake cylinder 202 from the pump 222. In the pressure hold mode for the front wheel brake cylinder 202, the second solenoid-operated valve device 214 is open to permit the pressurized fluid to be delivered from the pump 222 to the rear wheel brake cylinder 208 through the open valve device 214, as well as to the front wheel brake cylinder 204. Generally, the volume of the rear wheel brake cylinder 208 is smaller than that of the front wheel brake cylinder 202. When the pressure hold mode is established for the front wheel brake cylinder 202, therefore, the braking pressure in the rear wheel brake cylinder 208 is increased at a relatively high rate, while the braking pressure in the front wheel brake cylinder 202 is increased at a relatively low rate.
While the conventional braking system illustrated in FIG. 9 has the advantages described above, it has a drawback that it is difficult to control the rate of change of the braking pressure of each wheel brake cylinder as needed or desired.